The present invention relates to a control system for an AC motor/generator of secondary AC excitation type, or more in particular, to a control system for an AC motor/generator of the type adapted to control the output of the AC motor/generator, which is connected to an AC power system, in accordance with load variation of the power system.
The conventional AC power plant such as a pumped-storage power plant has problems that it is impossible to regulate the load during its operation in the pumping-up mode and the system efficiency changes depending on the amount of power generation or the head of water storage demanded by the system at operation in the power-generation mode or the pumping-up mode, respectively. In order to solve those problems, an attempt has been made to use an AC motor/generator or an induction motor/generator of secondary AC excitation type capable of variable speed operation, in place of the conventional synchronous machine.
On the other hand, in a typical AC power system, a plurality of synchronous machines are used in parallel, and each synchronous machine is operated as a motor or a generator in accordance with the load conditions of the system in order to stabilize the system. In case of occurrence of sharp variation of the load by an accident or the like, the synchronous machine is directly affected by the load variation and may result in step out. Various measures have been taken to prevent this step-out in the prior art. As described before, the induction motor/generator of secondary AC excitation type, which may be used in place of the synchronous machine in the pumped-storage power plant, is free from stepping-out due to its structural characteristic. If it is possible to prevent the synchronous machine from stepping out by absorbing the load variation of the power system by the induction motor/generator, the induction motor/generator provided with the pumped-storage power generation plant may be used conveniently for that purpose.